Researchers have for years discussed the merits of pumping water-based sulphate spray into the sky to reflect and scatter the Sun’s energy — essentially, mimicking the cooling caused by volcanic eruptions. Like most kinds of geoengineering, the idea is highly controversial and so far untested.

But if anyone does try this ‘solar-radiation management’, then it may be safer to use dusts of solid, nanometre-sized particles, suggests a team of scientists from Harvard University in Cambridge, Massachusetts. In a paper published in Atmospheric Chemistry and Physics1, they calculate that nanoparticles of diamond or alumina (aluminium oxide) could be more effective and less environmentally damaging than sulphates. And although diamond dust is expensive, it is not completely out of the question, the researchers argue.

“Our paper is really geared towards removing the mindset that it has to be sulphate that’s used to do solar radiation management,” says Debra Weisenstein, an atmospheric modelling expert at Harvard and one of the study’s authors.

Sulphate’s side-effects

Other researchers have proposed spraying solid dusts before2. But the latest study is the first to model the particles’ effects in detail, Weisenstein says, by examining how they interact — both physically and chemically — with different substances in the atmosphere, and making the comparison with sulphates.

In the atmosphere, sulphates lead to the production of sulphuric acid, which damages the ozone layer. By absorbing certain wavelengths of light, they also heat up the lower stratosphere; that in turn could affect air-circulation patterns and climate. Sulphates would also diffuse light, an effect that could boost plant growth but would lower the power output of solar panels3.

Alumina and diamond dust both lead to fewer problems, says Weisenstein. “You could have significantly less impact on ozone, less heating of the stratosphere and less of an increase in diffuse light at Earth’s surface,” she says. That is because alumina and diamond do not result in the production of sulphuric acid, and they scatter and absorb particular wavelengths of light in a different way.

Besides analysing environmental effects, the paper also shows that, pound for pound, alumina dust would achieve a similar cooling effect to that of sulphate sprays — but that diamond dust would be at least 50% more effective.

Diamonds in the sky

Of course, spraying diamond dust into the sky would ring up a hefty bill. Diamond dust is less expensive than cut gemstones: tiny synthetic diamond particles are now available at less than US$100 per kilogram, the Harvard researchers note. But based on their paper’s results, offsetting just a few percent of the energy trapped by human-emitted greenhouse gases would take hundreds of thousands of tonnes of dust annually. Although the Harvard researchers stress that they didn’t do a detailed cost analysis, at current prices that would still require billions of dollars each year.

However, Weisenstein is adamant that the ultimate cost would be lower. “Once this can be scaled up to make the right quantities, you assume the price is going to drop,” she says. “Trying to estimate based on how much diamond costs currently is not particularly useful.”

And David Keith, a climate scientist also at Harvard and another of the paper’s authors, says he does not think even today’s costs would be prohibitive. By 2065, he says, with 10 billion people on the planet, the cost might be on the order of $5 per person to pump up some 450,000 tons of diamond dust.

Still, the Harvard team is focusing on alumina right now, Weisenstein says, because it’s easier to make and its chemical behaviour has been better studied.

The scientists warn, however, that both alumina and diamond nanoparticles carry unknown risks. Sulphates are reasonably well understood, thanks to research on volcanoes. By contrast, the chemistry of the solid particles — such as how their surfaces catalyse chemical reactions — is not as clear, although the Harvard researchers are doing lab tests to remedy that.

The study “strongly suggests” that such solid dusts could significantly lower some of the risks associated with sulphates, says Matthew Watson, a volcanologist at the University of Bristol, UK, who was the principal investigator on one cancelled small-scale geoengineering experiment, the Stratospheric Particle Injection for Climate Engineering project, or SPICE. But he suspects that the unknown risks and lack of any natural analogue will make solid dusts even less popular with the public than are sulphate sprays.

2. Life is tough but you are tougher.

“I have heard there are troubles of more than one kind. Some come from ahead and some come from behind. But I’ve bought a big bat. I’m all ready you see. Now my troubles are going to have troubles with me!” ~ Dr. Seuss

3. It’s not about what it is, it’s about what it can become.

“It’s not about what it is, it’s about what it can become.” ― Dr. Seuss

4. If you never did, you should.

“If you never did, you should. These things are fun and fun is good!” ~ Dr. Seuss

5. There’s no limit to how far you can go.

“So be sure when you step, step with care and great tact. And remember that life’s a Great Balancing Act. And will you succeed? Yes! You will, indeed! (98 and ¾ percent guaranteed) Kid, you’ll move mountains.”~ Dr. Seuss

“There’s no limit to how much you’ll know, depending how far beyond zebra you go.” ~ Dr. Seuss

7. You can steer yourself any direction you choose.

“You have brains in your head. You have feet in your shoes. You can steer yourself any direction you choose. You’re on your own. And you know what you know. And YOU are the one who’ll decide where to go….” ~ Dr. Seuss

10. If no one cared an awful lot, not much would get done.

“Life’s too short to wake up with regrets. So love the people who treat you right, forgive the ones who don’t and believe that everything happens for a reason. If you get a chance, take it. If it changes your life, let it. Nobody said it’d be easy, they just promised it would be worth it.” ~ Dr. Seuss

16. Adults are just obsolete children.

“So be sure when you step. Step with care and great tact and remember that life’s a Great Balancing Act.”

18. A person’s a person, no matter how small.

“A person’s a person, no matter how small.” ~ Dr. Seuss

19. Nonsense wakes up the brain cells.

“I like nonsense, it wakes up the brain cells. Fantasy is a necessary ingredient in living, it’s a way of looking at life through the wrong end of a telescope. Which is what I do, and that enables you to laugh at life’s realities.” ~ Dr. Seuss

20. Think and wonder, wonder and think.

“Think left and think right and think low and think high. Oh, the thinks you can think up if only you try!” ~ Dr. Seuss

21. You are you. Now, isn’t that pleasant?

“Today you are You, that is truer than true. There is no one alive who is Youer than You.” ~ Dr. Seuss

22. Be awesome! Be a book nut!

“The more that you read, the more things you will know. The more that you learn, the more places you’ll go.” ~ Dr. Seuss

24. It is fun to have fun but you have to know how.

“From there to here, and here to there, funny things are everywhere.” ~ Dr. Seuss

“Look at me! Look at me! Look at me NOW! It is fun to have fun But you have to know how.” ~ Dr. Seuss

25. You’re never too old, too wacky, too wild, to pick up a book and read to a child.

“You’re never too old, too wacky, too wild, to pick up a book and read to a child.” ~ Dr. Seuss

26. Christmas doesn’t come from a store.

“And the Grinch, with his Grinch-feet ice cold in the snow, stood puzzling and puzzling, how could it be so? It came without ribbons. It came without tags. It came without packages, boxes or bags. And he puzzled and puzzled ’till his puzzler was sore. Then the Grinch thought of something he hadn’t before. What if Christmas, he thought, doesn’t come from a store. What if Christmas, perhaps, means a little bit more.” ~ Dr. Seuss

27. All creatures should be free.

“And turtles, of course… all the turtles are FREE. As turtles and, maybe, ALL creatures should be.” ~ Dr. Seuss

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While certainly not as dramatic as bursting into flames, for some people, sudden exposure to sunlight produces an unexpected reflex – they sneeze. Chances are this happens to you, or one of your friends. It’s called the ‘photic sneeze reflex’ and is more common than you’d expect, occurring in 17 to 35 percent of the world’s population, according to informal surveys. But what causes it?

Sneezing can’t really be controlled – it’s one of the body’s reflexes, and is typically associated with irritation in the nose. From here the signal is sent via neural pathways to the brain, resulting in a powerful release of air through your mouth and nose, which not only helps expel mucous or irritants from the nasal passages as fast as possible, but also contracts a bunch of muscles in the body, including the eyelids and the trachea.

When it comes to sun sneezing, even Greek philosopher Aristotle famously noticed the phenomenon and mentioned it in the ‘Nose’ chapter of his Book of Problems: “Why does the heat of the Sun provoke sneezing, and not the heat of the fire?”

However, the photic sneeze reflex has nothing to do with heat, and instead appears to be the result of crossed wires somewhere along the trigeminal nerve. Also known as the fifth cranial nerve, it’s the largest and most complex paired nerve in the head, with three major branches leading to the eyes, nasal cavity, and the jaw. It’s a crowded place in terms of nervous signalling, so it’s not surprising that the trigeminal nerve would occasionally get the reflexes wrong. Bright light causes your pupils to contract, so that signal might be mistakenly sent to the nose as well.

Another nervous system-related hypothesis states that these Sun-related sneezes might occur thanks to ‘parasympathetic generalisation’: a process that occurs when one part of the parasympathetic nervous system – such as the pupil of the eye – is excited by a stimulus and happens to activate other parts of the system as well – such as the membranes in the nose.

Whichever nervous system misfire is the exact cause of the problem, researchers have figured out the underlying genetics of sun-sneezing. “[T]he reflex is now also known by the hilariously apt acronym Achoo, which stands for Autosomal Dominant Compelling Helio-opthalmic Outburst,” writes Jason Goldman at BBC. “‘Autosomal’ because the affiliated gene is located on one of the non-sex-linked chromosomes, and ‘dominant’ because you only need to inherit it from one of your parents to express the trait.”

Even though commonly associated with the Sun, photic sneeze reflex can also happen thanks to any other sudden light exposure, such as a flashlight in a dark room. And because it’s a harmless condition, you can go on and try this on your sneeze-afflicted friend – just don’t tell them it was our idea.